1. Field of the lnvention
The present invention is directed to a process for the production of matte, non-blocking, thin-walled molded articles, films or tubes from polyurethane elastomers, the elastomers produced therefrom and their use for back-foaming with polyurethane foams.
2. Description of the Prior Art
Thermoplastic polyurethane elastomers have long been of technical importance on account of their combination of valuable mechanical properties and high chemical resistance and the ease with which they can be worked up economically by thermoplastic processes (see Becker/Braun, Kunststoff-Handbuch, Volume 7; Polyurethane, Carl Hanser Verlag, Munich-Vienna 1983, pages 428 to 440). When thermoplastic processes are applied to polyurethanes which have not been cross-linked (especially soft polyurethanes with Shore A hardnesses below 95), difficulties arise in the production of thin-walled articles due to severe blocking and sticking when attempts are made to remove the products from the mold. This difficulty occurs particularly in the production of films, thin-walled tubes and molded articles from linear, thermoplastic PU elastomers (TPU) by extrusion, calendering or blow molding. Also, the considerable surface tackiness of soft TPU foils often severely restricts their possible uses.
The usual addition of so-called internal release agents such as stearylamines, stearyl urethanes or stearyl urea and the less commonly practiced addition of inorganic anti-blocking agents such as chalk, talcum, diatomaceous earths and the like are equally unsatisfactory for solving this problem. Further, the last mentioned fillers generally cause a deterioration in the properties of the TPU (e.g. loss of resistance to hydrolysis and appearance of processing difficulties such as the formation of blisters on the surface due to moisture adhering to the fillers). Moreover, the internal release agents melt at the high processing temperature and are liable to migrate from the molded product when used at the high concentrations required. In particular, they give rise to difficulties when TPU foils are bonded or glued or back-foamed.
It is an object of the present invention to provide non-blocking, matte, thin-walled molded articles having wall thicknesses below about 2 mm, preferably about 5 .mu.m to 500 .mu.m, in particular about 20 .mu.m to 200 .mu.m, by the extrusion, calendering or blow-molding of substantially linear, thermoplastic polyurethanes without the usual disadvantages of soft, thermoplastic polyurethanes and with the advantage that the polyurethanes used would form a rapidly solidifying melt which would readily separate from the mold and give rise to matte, homogeneous, thin-walled molded articles such as sheets, films or tubes which would have a dry, non-blocking surface and would therefore be immediately ready for further processing, e.g. films or tubes could be immediately rolled up. It is also an object to provide thin-walled molded products, in particular films or tubes, which do not stick together or block and yet when used as outer skin, film or molded structure adhere firmly to polyurethane foam systems foamed up inside them or on their suface so that a firm bond is obtained between the film and the foam system.
This object was surprisingly achieved by using substantially linear thermoplastic polyurethanes obtained from mainly relatively high molecular, difunctional compounds containing isocyanate reactive groups and having molecular weights of 400 to about 10,000 and melting points below about 60.degree. C., preferably relatively high molecular weight diols in which relatively small quantities of selected, high melting polyisocyanate polyaddition products ("polyadducts") are dispersed and remain dispersed in a finely divided state in the polyurethane elastomer matrix after processing.